Magnesium anodisation methods

ABSTRACT

This invention relates to a method of anodising magnesium material which includes anodising the magnesium while it is immersed in an aqueous electrolyte solution having a pH above 7, and in the presence of a phosphate, the electrolyte solution also containing a sequestering agent. The method may further include the provision of a plasma suppressing substance within the electrolyte solution. Furthermore, the electrolyte solution may also preferably include a tertiary amine such a TEA, and the current passed through the electrolyte solution may preferably be a straight DC current.

TECHNICAL FIELD

This invention relates to magnesium anodising systems and methods.Throughout this specification, the terms “magnesium”, “magnesium metal”and “magnesium material”, may be used interchangeably, and are all to beunderstood to refer to or include magnesium metal and/or magnesiumalloy(s) and/or mixtures thereof, and/or any articles or compoundscomprising or including magnesium.

BACKGROUND ART

Magnesium is a very light, yet strong metal and is finding increasingacceptance for metal die castings, particularly where weight savings aredesired. In addition, its property of shielding electromagneticradiation is causing it to be of interest as a replacement for plasticsin applications such as computers and mobile telephones. However, it isa reactive metal and corrosion, whether general or by galvanic effects,is a major problem.

A number of methods for applying a protective anodic oxide film onmagnesium material have been available. These have sought to imitate thewell established processes available for coating aluminium and itsalloys, however achieving the same result on magnesium articles has beenextremely difficult.

The anodisation of aluminium and its alloys is often conducted insulphuric acid in which the oxide layer formed is slightly soluble. Asthe film builds outwards from the metal substrate, its rate of builddecreases, so ultimately there is an equilibrium point at which the rateof dissolution is equal to that of further film growth. The dissolutionof the film causes the formation of pores through which the ionicmigration necessary to the electrochemical oxidation of the metal takesplace. Without these pores only very thin films would be possible. Afterthe electrochemical oxidation process is complete, the pores are scaled.Sealing of anodised aluminium can be achieved with hot water or simpleinorganic chemical solutions.

Clearly an analogous process involving magnesium would attempt tosimulate these features. However, because of the tendency of the formingfilm to crack and break due to the imposed tensile stresses, there arecomplications. Also, the use of an acidic solution to anodise magnesiumis fraught with serious difficulties as magnesium is rapidly attacked bymost common acids. Therefore, anodisation of magnesium should preferablytake place in alkaline solutions.

One method of anodising magnesium relies on this property to create arough, very porous layer which may form a base for paint or othersurface coatings to be applied afterwards. Commonly, such an anodic filmmay be formed in an electrolyte of high pH, containing alkalihydroxides. The process proceeds by means of sparking, which sparkingforms a sintered ceramic oxide film as the metal substrate is coated.

However, the forming of a sintered ceramic oxide film, through sparking,is not always desirable as the film is often brittle, uneven, and/orlacks uniformity.

A number of proprietary methods for anodisation of magnesium exist whichseek to avoid this problem and hence create a stronger and/or moreuniform film.

In PCT/NZ96/00016 (WO 96/28591) (Barton) there is disclosed a viableprocedure for anodising magnesium or magnesium alloys. It involvesanodising the material in an ammonia containing electrolyte solution.The presence of some phosphate compounds in the solution is alsodisclosed. Enhancements of such a Barton procedure are disclosed inPCT/NZ98/00040 (WO98/42892) (MacCulloch et al).

For environmental reasons arising from the emanation of ammonia and alsotaking into account potential problems associated with the disposal ofammonia-containing electrolytes and process washings, a process isdesirable beyond those aforesaid where no ammonia or ammonium salts arepresent in the electrolite. However, the absence of ammonium compoundsimposes difficulties in the functioning of the process in the areas ofanodic polarisation, repeatability and film quality.

In PCT/NZ01/00215 (WO 02/28838 A2) there is disclosed another viableprocedure for anodising magnesium or magnesium alloys which do away withammonia-containing electrolytes. This method includes anodising themagnesium material while it is immersed in an aqueous electrolytesolution having a pH above 9, and in the presence of a phosphate (orphosphate ions). The solution also preferably includes a buffering agentsuch as a tetra-borate to maintain the pH of the solution above 9. Thereare also described pre-treatment steps prior to anodising.

Whilst the methods and apparatus described in PCT/NZ01/00215 result in aviable procedure for anodising magnesium, the solutions contain boron(or a borate) which is not always desirable as it can be environmentallyharmful if not disposed of properly after use. Furthermore, some of thepre-treatment steps described are somewhat involved. It would thereforebe desirable if there was a viable procedure for anodising magnesium ormagnesium alloys which used an electrolyte that preferable did notcontain ammonia and/or boron/borate and/or did not require the use ofsuch involved pre-treatment step(s).

Furthermore, many procedures for anodising magnesium necessarily involvethe use of a pulsed DC current, which requires the use of specialisedand expensive rectifiers. It would also be desirable therefore if therewas available a viable procedure which produced the required or desiredresults using straight or flat waveform DC (referred to herein as“straight DC”).

It is therefore an object of the present invention to address theforegoing problems or at least to provide the public with a usefulchoice.

Further aspects and advantages of the present invention will becomeapparent from the ensuing description that is given by way of exampleonly.

DISCLOSURE OF INVENTION

According to one aspect of the present invention there is provided amethod of anodising magnesium material which includes anodising themagnesium material while it is immersed in an aqueous electrolytesolution having a pH above 7 and in the presence of a phosphate, theelectrolyte solution also containing a sequestering agent.

According to another aspect of the present invention there is provided amethod, substantially as described above, wherein the phosphate is analkali metal phosphate.

According to another aspect of the present invention there is provided amethod, substantially as described above, wherein the pH is in the rangeof 10.2-11.0.

According to another aspect of the present invention there is provided amethod, substantially as described above, wherein the electrolytesolutions contains an alkali metal hydroxide.

According to another aspect of the present invention there is provided amethod, substantially as described above, wherein the alkali metalhydroxide is KOH.

According to another aspect of the present invention there is provided amethod, substantially as described above, wherein the electrolytefurther includes a plasma suppressing substance.

According to another aspect of the present invention there is provided amethod, substantially as described above, wherein the electrolytefurther includes an amine.

According to another aspect of the present invention there is provided amethod, substantially as described above, wherein the amine is TEA.

According to another aspect of the present invention there is provided amethod, substantially as described above, wherein the sequestering agentis in the form of ethylene diamine tetramethylene phosphonic acid.

According to another aspect of the present invention there is provided amethod, substantially as described above wherein the current passedthrough the electrolyte solution is a pulsed DC current.

According to another aspect of the present invention there is provided amethod, substantially as described above, wherein the current passedthrough the electrolyte solution is a straight DC current.

According to another aspect of the present invention there is provided amethod, substantially as described above, wherein the anodising of themagnesium material follows a pre-treatment designed to prepare themagnesium material for anodisation.

According to another aspect of the present invention there is provided amethod, substantially as described above, wherein the anodising of themagnesium material follows one or more of the pre-treatment stepsdescribed in WO 02/28838 A2.

According to another aspect of the present invention there is provided amethod of anodizing magnesium material, substantially as describedabove, wherein the anodizing of the magnesium material follows apre-treatment designed to prepare the magnesium material foranodization.

According to another aspect of the present invention there is provided amethod, substantially as described above, wherein the pre-treatmentincludes one or more of the following sub-steps:

-   (a) a cleaning step,-   (b) an etching step,-   (c) a surface activation step.

According to another aspect of the present invention there is provided amethod, substantially as described above, wherein there is a cleaningstep before and after the etching step.

According to another aspect of the present invention there is provided amethod, substantially as described above, wherein the cleaning stepincludes an immersion of the magnesium material into a solutioncontaining caustic soda.

According to another aspect of the present invention there is provided amethod, substantially as described above, wherein the etching stepincludes an immersion of the magnesium material into a solutioncontaining at least one acid.

According to another aspect of the present invention there is provided amethod, substantially as described above, wherein the acid is nitricacid or phosphoric acid.

According to another aspect of the present invention there is provided amethod, substantially as described above, wherein the etching stepincludes an immersion of the magnesium material into a solutioncontaining DEOXALUME®

According to another aspect of the present invention there is provided amethod, substantially as described above, wherein the surface activationstep includes an immersion of the magnesium material into a solutioncontaining a source of fluoride ions.

According to another aspect of the present invention there is provided amethod, substantially as described above, wherein the surface activationstep includes an immersion of the magnesium material into a solutioncontaining a source of fluoride ions and an acid.

According to another aspect of the present invention there is provided amethod, substantially as described above, wherein the surface activationstep includes an immersion of the magnesium material into a solutioncontaining potassium fluoride and nitric acid or phosphoric acid.

According to another aspect of the present invention there is provided amethod, substantially as described above, wherein the surface activationstep includes an immersion of the magnesium material into a solutioncontaining ammonium bifluoride.

According to another aspect of the present invention there is provided amethod, substantially as described above, wherein the surface activationstep includes an immersion of the magnesium material into a solutioncontaining DEOXALUME®.

According to another aspect of the present invention there is provided amethod, substantially as described above, wherein between each substepthere is optionally a rinsing step.

Generally speaking, the methods described in Barton and MacCulloch donot usually require a thorough cleaning of the magnesium metal prior tothe anodization process. This is because the electrolytes described inBarton and MacCulloch contain ammonia, which is very effective in anelectrolyte solution for the purposes of anodizing magnesium materialregardless (to a certain extent) of the cleanliness of the magnesiummaterial prior to anodizing.

We have found that by pre-treating the magnesium material, prior toanodization, we are able to achieve a virtually equivalent result asregards the quality of the anodic film formed (as regards -uniformity,strength and evenness etc) as the processes described in Barton andMacCulloch, but without requiring ammonia to be present in theelectrolyte. This is desirable given that we are no longer faced withthe environmental/health problems associated with using and disposing ofammonia containing electrolytes, as described previously.

Generally speaking, surface cleaning and preparation of metal substratesfor an electro chemical process is a complex field and thepre-treatments presented below are therefore given by way of exampleonly. Moreover, in some situations where components are heavily soiled,for example with die lubricants, or have surface corrosion, specialcleaning steps additional or alternative to those listed herein may berequired. Alternatively, good quality components with clean surfaces mayrequire fewer or less rigorous cleaning steps.

The at least one pre-treatment steps described above as (a), (b), (c)may be undertaken in any order and/or may be repeated as required or asdesired, or as dictated by the condition of the magnesium material to bepre-treated and subsequently anodized. Furthermore, and again dependingupon the condition of the magnesium material, only one or two (or three)of the pretreatment sub-steps may be utilized.

Preferably, the cleaning step may be followed by the etching step, andsubsequently followed by the surface activation step. Alternatively, oradditionally, there may be provided an additional cleaning step after,the etching step, but prior to the surface activation step.

The cleaning step may involve the immersion of the magnesium materialinto an appropriate cleaning solution.

Preferably, the cleaning step may involve the immersion of the magnesiummaterial into a solution which includes caustic soda. Any suitableconcentration may be utilized as required or as desired, or as dictatedby the condition of the magnesium material to be cleaned.

Preferably, the caustic soda may include sodium hydroxide at aconcentration of between 10-50% w/v. A concentration of approximately30% w/v may be particularly suitable.

The magnesium material may be immersed in the cleaning solution for anylength of time, as required or as desired, or as dictated by thecondition of the magnesium material. Preferably, the immersion time maybe between 2-12 minutes, with approximately 5 minutes being particularlysuitable.

The caustic soda solution may be at any suitable temperature, asrequired or as desired, or as dictated by the condition of the magnesiummaterial. Preferably, the solution may be at a temperature of between50-95° C., with a range of 70-85° C. being particularly suitable.

Preferably, after the cleaning step the magnesium material may berinsed, and preferably with water. De-ionized water may be particularlysuitable.

The etching step may preferably include the immersion of the magnesiummaterial into a solution containing at least one acid. Any suitable acidor acids may be utilized as required or as desired. Examples includenitric acid and phosphoric acid.

Any suitable concentrations of acid may be utilized as required or asdesired. For example, if the acid used is nitric acid, it maypreferablybe of a concentration of approximately 0.4-0.8 M, with aparticularly suitable range being 0.5-0.6 M.

Alternatively, the magnesium material may be immersed into a solutioncontaining DEOXALUME®. which is a proprietary product manufactured byHenkel Corporation. If DEOXALUME®. is used, it may preferably be dilutedto, approximately a 10% concentration.

The etching step serves to remove surface layers of the magnesiummaterial which assists in the anodization process.

The magnesium material may be immersed in the etching solution for anylength of time, as required or as desired or as dictated by the state ofthe magnesium material. For example, if phosphoric acid or nitric acidwere to be used a time of approximately 30 seconds to 4 minutes may besuitable. If DEOXALUME® is used, a time of approximately 10-30 secondsmay be suitable.

Similarly, the temperature of the etching solution may be in the rangeof 10-80° C., with a range of approximately 20-40° C. being particularlysuitable.

Preferably, the magnesium material may be rinsed after the etching step,and preferably with water. De-ionized water may be particularlysuitable.

Preferably, a further cleaning step, substantially as describedpreviously, may be undertaken after the etching step, and preferably afurther rinsing of the magnesium material, for example with de-ionizedwater, may follow the second cleaning step.

In this specification, the term “phosphate” is understood to include orrefer to, collectively or singularly, either a phosphate or a source ofphosphate ions. Furthermore, the term TEA is understood to refer to thetertiary amine Tri-ethanolamine.

The method of anodising magnesium material may include the step ofanodising the magnesium material while it is immersed in an aqueouselectrolyte solution having a pH above 7, and in the presence of aphosphate and a sequestering agent.

The phosphate may include an ortho-phosphate and/or a pyro-phosphate.

Any suitable source of phosphate may be utilised in the solution. Forexample, an alkali metal phosphate such as sodium dihydrogen orthophosphate. Alternatively, or additionally, the phosphate may be providedby a phosphoric acid, or salt thereof.

Any suitable concentration of phosphate may be utilised as required oras desired, and experimental trial and error will enable the optimum ordesired range of concentration to be ascertained. In general termshowever phosphate concentrations of the order of 0.02M to 0.1M may beparticularly suitable. It Is to be understood and appreciated that thisrange is given by way of example only, and concentrations of phosphateoutside this range is also within the scope of the present invention.

The pH may preferably be greater than 9, and, more specifically, a pH inthe range of 10.2-11+ is found to be particularly suitable.

Any suitable base may be utilised to reach and maintain the desired pH.For example, the electrolyte solution may be provided with a source ofhydroxide ions, for example an alkali metal hydroxide such as KOH orNaOH.

Any suitable concentrations of base may be utilised as required in orderto reach a preferred or desired pH.

The electrolyte solutions may also include a plasma suppressingsubstance. The role of the plasma suppressing substance is primarily toreduce the tendency for plasma discharges to form at defect sites onarticles being anodised. An example of a suitable plasma suppressingsubstance may be an acrylic modification of maelic acid. A furtherexample is the product P80.RTM., which is a compound manufactured byCyanamid Corporation of the United States and which is a copolymer ofallyl sulfonic acid and maleic anhydride, that is to say apolyacrylamide, as disclosed, for example in, U.S. Pat. No. 4,810,405 toWaller, et al. issued on Mar. 7, 1989, entitled Rust removal andcomposition thereof, and U.S. Pat. No. 5,062,962 to Brown et al. issuedon Nov. 5, 1991, and entitled Methods of controlling scale formation inaqueous systems.

Any suitable amounts or concentrations of the plasma suppressingsubstance may be utilised as required or as desired. For example, aconcentration in the range of 100 to 400 ppm may be suitable, althoughconcentrations of the plasma suppressing substance outside of this rangeare also within the scope of the present invention.

The electrolyte solution may preferably include a sequestering agent.One role of the sequestering agent is to bind any loose or superfluousions (usually metal ions) so that they cannot react and, for example,form white powder deposits and the like.

Furthermore, we have found that the use of a sequestering agent togetherwith an amine such as TEA produces a surprising and advantageous resultin that the anodisation of the magnesium material is found to proceedsatisfactorily with only a straight DC current.

Any suitable sequestering agent ma)y be utilised, for example ethylenediamine tetramethylene phosphonic acid or DEQUEST® 2066 manufactured byHenkel Inc of the United States. Any suitable concentration range may beutilised and this may be determined by trial and experimentation.However, a concentration range of the order of 0.002M to 0.02M may beparticularly suitable. Concentrations outside of this range are howeveralso deemed to be within the scope of the present invention.

The electrolyte solution may also preferably include an amine, and moreparticularly a secondary or tertiary amine.

It is found that TEA is particularly suitable as it appears to work withthe sequestering agent to produce the surprising result referred topreviously.

Again, the concentration of the TEA may be any required or desiredlevel, although a concentration in the range of 40-150 g/l may beparticularly suitable. Again, a concentration outside of this range isalso considered to be within the scope of the present invention.

The use of DC currents generally for anodising magnesium are well knowand, for example, are described in considerable detail in WO 02/28838A2.

The voltage applied to the electrolyte solutions may preferably be adirect current (DC). It is found that either a pulsed or a DC currentmay be suitable for use with the methods of the present invention.However, when the electrolyte solutions contains both an amine such asTEA and a sequestering agent such as DEQUEST® 2066 it is found that theanodisation of the magnesium material proceeds quite satisfactorily withjust the use of straight DC current. This is of advantage and ofcommercial significance as a straight DC current does not require theuse of expensive and/or specialised rectifiers and the like which arerequired to produce a pulsed current.

Preferably, the magnesium material may be pre-treated and or cleanedprior to the anodising of same. Any suitable pretreatment and/orcleaning of the magnesium material may be utilised as required or asdesired, or as dictated by the condition or state of the magnesiummaterial. Preferably, and for example, the anodising of the magnesiummaterial may follow one or more of the pre-treatment steps described inWO 02/28838 A2.

It is to be understood and appreciated however that the possiblepre-treatment steps outlined in WO 02/28838 A2 are by no means the onlypossible pre-treatment processes or steps. For example, in situationswhere the magnesium material is heavily soiled with, for example, dielubricants or surface corrosion, additional or alternative cleaning orpre-treatment steps may be required and/or these steps may need to berepeated. Similarly, if the magnesium material is particulars cleanand/or of good quality, it may require fewer or less rigorous cleaningor pre-treatment steps.

We have also found that the use of TEA and/or the sequestering agentallows less intensive pre-treatment or cleaning steps to be undertakenin order to prepare the magnesium material satisfactorily for theanodising process.

The types of apparatus and/or conditions under which magnesium materialshould preferably be anodised are well documented, for example theprocesses described in the prior art already referred to. However, ageneral overview of the apparatus and techniques to be utilised is asfollows.

The anodic reaction takes place in a vessel in which the article to beanodised is connected to an electrically-conductive rack and immersed inthe electrolyte. Generally, the rack will be coated in plastic exceptfor small contact areas where it forms an electrical connection to thearticle being anodised. Where the rack is composed of a material thatwill passivate under the electrical conditions of tile anodisingprocess, it is not necessary to coat the rack with an insulator, but itmay be desirable to do so for improved efficiency.

In general it is advantageous for the vessel containing the electrolyteand the article to be anodised to be made of insulating plastic,provided that electrically conductive counter-electrodes are inserted inthe tank, most commonly in the sides. It is desirable that these beinert chemically, preferably of stainless steel, type 316. Although itis possible to use counter-electrodes composed of alternativesubstances, for example, aluminium, this is undesirable since in anothermodification of the process, a reverse polarity voltage is applied tothe article resulting in a brief, anodic polarisation. Stainless steelhas the advantage of being inert under these conditions whereasaluminium would anodise, preventing the proper functioning of thestandard cycle.

The electrolyte is operable over a broad temperature range, from aroundzero to its boiling point, but the process operates optimally over arange 20-60° C. The voltage applied to the electrolyte is normallydirect current. The output produced by a rectified three phase powersupply, comprising a voltage of constant polarity fluctuating byapproximately 5% is suitable, as is smoothed DC. Modified waveforms, forinstance, pulsed or superimposed AC voltages may also be employedalthough these result in different film thickness and othercharacteristics than that normally obtained from direct currentanodisation.

When an anodic voltage is first applied to the article to be anodisedthe electrical resistance is low but this progressively increases as aninsulating anodic film forms on the surface. The result is an increasingvoltage when anodising current is held constant. The process is normallycontrolled by means of a constant current, preferably in the range 50A/m² to 500 A/m² and optimally around 200 A/m². When operated at 200A/m², the imposed voltage may be expected to reach 200 volts after twoto three minutes, and for a commercially-useful coating, the voltage mayreach an ultimate limit of 230 to 270 volts. Very thin films, suitablefor some applications may be achieved using lower voltages. The filmcontinues to build if the voltage is held constant on attaining acertain limit, for example, 220 volts, and as this takes place, thecurrent dwindles.

Usually the process requires less than 5 minutes. As the voltage reachesa range of 200 to 270 volts, it is quite common for localised plasmadischarges to form, particularly at defect sites. These plasmas arecharacterised by a changed coating morphology and possibly there areassociated thermal effects on underlying metal structures. Suppressionof the initiation of such plasma discharges has been achieved with theaddition of the plasma suppressing substances referred to previously.

Since power supplies vary in their characteristics and the ultimatevoltage achieved for an equivalent film thickness is highly dependent onaspects such as ripple percentage, the presence or absence of pulses andother electrical characteristics, the voltages stated above are nothingmore than indications. The process is operable over a broad range ofvoltages and current densities.

In a modification of the standard process described in the foregoingparagraphs, a brief cathodic voltage may be applied to the article priorto anodisation. This is usually current controlled and results in arelatively low voltage, typically less than 20 volts, and considerablegassing from the article in the electrolyte. Such a cathodic cycle isnot known to influence the chemical composition of the surface of thearticle to be anodised, but may assist with preparation of a clean anduniform surface for anodisation.

Since the market for magnesium or magnesium alloy articles ispredominantly die cast components, the characteristics of articles thatwill be anodised is rather different to the aluminium market in whichanodised components are often extruded or are flat profiles. Many diecast articles feature complex shapes and manifest extensive surfacedefects, including inclusions, porosity, flow marks and shapes thatcreate difficulties for electrochemical processing by reason of airentrapment or flow stagnation.

It is desirable that the anodising electrolyte has efficient circulationboth for reasons of maintaining uniform electrolyte composition and heatremoval. Stagnant flow may be minimized by the use of ultrasoniccleaning devices during anodisation. The use of ultrasonic cleaningduring anodisation results in a clean, smooth anodic film. It appearsthat ultrasonic energy reduces the boundary layer on the surface of theforming film and improves ionic transfer to the bulk electrolyte. Thereis an additional benefit in that loosely adherent particles, forexample, inclusions in die cast components, are removed more readily.

Ultrasound use is not limited to the anodising electrolyte, and may alsobe used to improve rinse or cleaning process efficiency. However, theapplication of ultrasound to cleaning processes is well established insuch processes.

Other means of improving electrolyte circulation where there may beproblems associated with flow stagnation, or air pockets developingunder submerged recesses, involve the use of flow adductors or rackswith inbuilt rotation or movement cycles. These techniques are sometimesobserved in other electrochemical processes. A rotary barrel system suchas is commonly employed in electroplating or chemical plating processesis not suitable since the anodic film formed during the processdisclosed herein is not electrically conductive.

A composite coating comprising many layers features many potentialproblems, including the expense of several processing stages and theaccumulated probability of failure from each of those steps. Plainly itis desirable to achieve the final result in as few steps as possible.Since the overall production rate is determined by the cycle time of theslowest process, time savings in processing lead to efficiency gainsoverall.

The methods disclosed by Barton and MacCulloch are optimally conductedat temperatures lower than 10° C., thereby requiring the use ofcompressive refrigeration to remove waste heat from the processsolution. This entails considerable capital expenditure and additionalenergy costs. For the purposes of the present invention, a cooling toweris sufficient for commercial production. The result is a significantsaving.

A common problem encountered in anodising magnesium articles arises fromthe fact that many magnesium articles are die cast rather than extruded,forged or rolled. Die-castings frequently manifest a range of defects.These include porosity, cracks, flow lines, inclusions, plaques ofexternally solidified material and others. As a tool steel die agesdefects arise from tool wear. Die-casting alloys are frequentlyheterogeneous, unlike the homogeneous solid solutions that arefrequently used for extrusion.

Accordingly, unusual anodising behaviour at defect sites may sometimesoccur. It is found that the sequestering agent, being added to theelectrolyte solution, suppresses the tendency for white powder depositsto form.

In some embodiments of the present invention, the electrolyte solutionmay include a buffering agent to maintain the pH and the desired levelor range. Any suitable buffering agent may be utilised, although atetra-borate may be particularly suitable. Moreover, an alkali metaltetraborate such as sodium tetraborate may be particularly suitable.

BEST MODES FOR CARRYING OUT THE INVENTION

Some examples of best modes for carrying out the invention are describedbelow.

-   1. An electrolyte was prepared as follows:    -   Sodium dihydrogen orthophosphate (NaH₂PO₄.2H2O)—6 g/l    -   Sodium tetraborate (Na₂B₄O₇.5H₂O)—30 g/l    -   Sodium hydroxide (NaOH)—approx 10 g/l    -   Ethylene diamine tetramethylene phosphonic acid (C₆H₁₆O₁₂N₂P₄)—3        g/l    -   The phosphate salt was dissolved in deionised water, and the        borate added slowly at a temperature of around 40° C. Sodium        tetraborate pentahydrate, as used in this example, is quite slow        to dissolve as there is a tendency for the formation of large,        slow-to-dissolve crystals. The pH was then adjusted upwards to        11.0 by adding sodium hydroxide solution. Finally, the organic        acid was added. Pre-cleaning steps comprising 2 minutes in 3.5%        nitric acid at ambient temperature, 5 minutes in 25% NaOH        solution at 80° C. and 5 minutes in 0.03M ammonium bifluoride at        40° C. Anodising was performed at 200 A/m², with the voltage        starting from zero and rising to around 230 volts before the        process was terminated. A uniform, smooth, powder-free film of        about 3-4 μm thickness was formed on the surface of articles of        the magnesium alloys AZ91D, AM60 and AZ31B.-   2. An electrolyte was prepared as follows:    -   Sodium dihydrogen orthophosphate (NaH₂PO₄.2H₂O)—6 g/l    -   Sodium tetraborate (Na₂B₄O₇.5H₂O)—30 g/l    -   Sodium hydroxide (NaOH)—approx 10 g/l    -   Ethylene diamine tetramethylene phosphonic acid (C₆H₁₆O₁₂N₂P₄)—3        g/l    -   Acrylic modified maleic acid (P80®, a proprietary compound of        the Cyanamid Corporation, USA)—200 ppm    -   The electrolyte was prepared as for example #1 above, with the        P80® component added after the organic acid. Pre-treatments were        as for the example above. The anodising was conducted at 200        A/m², with the voltage starting from zero and reaching about 250        volts. No tendency for plasma discharges was noticed even though        poor quality die cast samples were deliberately chosen for the        experiment. The anodic film was smooth and uniform, similar to        that described above.-   3. We then experimented with an electrolyte solution that had no    boron or borates and instead used TEA. Specifically, the electrolyte    contained:    -   ortho-phosphate ions    -   TEA    -   A suitable base giving a pH above 10.        It was found that this process worked with only a caustic-based        degreasing step as its pre-treatment. However, anodising in this        electrolyte specifically required the use of a specified pulse        DC current.-   3. The deposited coating was Mg₃PO₄. An electrolyte was prepared as    follows:

Phosphoric acid 75% 100 g/L Triethanolamine 99% 85 g/L PotassiumHydroxide solution 45% 210 g/L (pH = 11.2) Conductivity 70 mS at 20° C.

-   -   Anodising was carried out at 200 A/m² at 45° C. using a pulsed        waveform (10 ms on 10 ms off) for 3 min. The average voltage was        90 Volts with a peak voltage of 195 Volt.    -   The deposited anodic layer was a light grey and had a thickness        of 14 um.

An attempt to anodise a magnesium test plate in the same electrolyteunder the same conditions except that tile power supplied was continuousthree phase, unfiltered, full wave, rectified current did not produceany meaningful polarisation of the anode and hence no film wasdeposited.

However, when 4 g/L of “DEQUEST 2066” was added to the same bath afurther experiment showed that a good film of some 12-15 um could bedeposited using an equivalent continuous DC current. Subsequently thissolution with the DEQUEST® behaved in a similar way to Example 3 above,which used a pulsed current.

-   4. An electrolyte was prepared as follows:

Phosphoric acid 85% 90 g/L Triethanolamine 99% 90 g/L Dequest 2066 2 g/LSodium Hydroxide To achieve pH = 11.0 Conductivity 75 mS at 20° C.

Anodising was carried out at 300 A/m² at 45° C. using filtered DC for 2min. The average voltage was 70 Volts with an end voltage of 155 Volt.The deposited anodic layer was a light grey and had a thickness of 10um.

Triethanolamine is a preferred tertiary amine as it is odourless, hasgood solubility, a high boiling point, and a satisfactory dissociationconstant. Generally it has been observed that a high viscosity anodisingsolution is beneficial to film formation especially if this results fromthe employment of high molecular weight substituted tertiary orsecondary amines. An example was the use of 75 g/L of 1-di-ethyl amino2-propanol. The films produced were easily formed at low average voltageand at good current efficiency.

The addition of a small amount of a phosphonate such as “Dequest ” 2066or 2041 to the anodising bath allows the anodising process to proceedwith both pulsed waveforms and also filtered and unfiltered DC.

The following pre-treatnielt scheme was applied to both AZ91 and AM50alloys and was found to be beneficial in obtaining good polarnsation andan even coating.

-   -   a. Degrease in hot NaOH and detergent at 70° C. for 5 mins.    -   b. Rinse in water for 3 mins.    -   c. Soak in 2% ammonium bi-fluoride solution for 5 mins.    -   d. Water rinse.

Coating thickness and porosity can. to some degree, be controlled bychoosing various combinations of both current density and time. Forexample, a high current density for a short time will produce a lessporous film than a lower current density for a longer time given thatthe film thickness is the same in both cases.

When using pulsed waveforms similar to that shown in Example 3 the ratioof peak current to average current can be as high as 10:1. This could bedisadvantageous in some cases as the power supply must be over-designedfor relatively small average currents.

Potassium hydroxide is the preferred alkali.

A lower electrolyte pH in combination with the phosphonate additive wasfound to be beneficial in promoting anodic film formation on substratesthat had had high aluminium content due to segregation. This wasparticularly so if fluoride pre-treatment was used.

As an alternative to the cathodic treating of the magnesium materialprior to anodizing, the magnesium material may instead be pre-treated.The pre-treatment preferably includes the following steps, namely acleaning step, an etching step, and a surface activation step.

Preferably, the magnesium material is first subjected to a cleaning stepfollowed by the etching step, followed by a further cleaning step, andfollowed lastly by a surface activation step. Preferably, in betweeneach of the steps as just described, there is a rinsing step involvingthe rinsing of the magnesium material with de-ionized water. Thispre-treatment process is summarized in FIG. 2.

Some examples of best modes for carrying out the invention, utilizingthe pre-treatment step, are 10 described below:

-   1. An electrolyte was prepared as follows:    -   Sodium dihydrogen orthophosphate (Na₂H₂PO₄.2H₂O)—6 g/l    -   Sodium tetra-borate Na₂B₄O₇.5H₂O—30 g/l    -   Sodium hydroxide (NaOH)—approx 10 g/l    -   The phosphate salt was dissolved in deionized water, and the        borate added slowly at a temperature of around 40° C. Sodium        tetra-borate pentahydrate, as used in this example, is quite        slow to dissolve as there is a tendency for the formation of        large, slow-to-dissolve crystals. The pH was then adjusted        upwards to 11.0 by adding sodium hydroxide solution.    -   Pre-treatment steps were as follows:    -   (i) 5 minutes in 25% NaOH solution at 80° C.    -   (ii) Rinsing with de-ionized water.    -   (iii) 2 minutes in 3.5% nitric acid at ambient temperature.    -   (iv) Rinsing with de-ionized water.    -   (v) 5 minutes in 25% NaOH solution at 80° C.    -   (vi) Rinsing with de-ionized water.    -   (vii) 5 minutes in 0.03M ammonium bifluoride at 40° C.    -   (viii) Rinsing with de-ionized water.    -   Anodizing was then performed in the electrolyte described above        at 200 A/m², with the voltage starting from zero and rising to        around 230 volts before the process was terminated. The anodic        film formed was smooth and uniform.-   2. An electrolyte was prepared as follows:    -   Sodium dihydrogen orthophosphate (Na₂H₂PO₄.2H₂O)—12 g/l    -   Sodium tetra-borate Na₂B₄O₇.5H₂O—15 g/l    -   Sodium hydroxide (NaOH)—approx 15 g/l    -   The electrolyte was prepared as for the previous example.        Pre-treatments were as for the previous example. The anodizing        was conducted at 200 A/m², with the voltage starting from zero        and reaching about 230 volts. A smooth, uniform film, similar to        that described in example #1 above resulted.

Aspects of the present invention have been described by way of exampleonly and it should be appreciated that modifications and additions maybe made thereto without departing from the scope thereof, as defined inthe appended claims.

1. A method of anodizing magnesium material comprising: anodizing themagnesium material while it is immersed in an aqueous electrolytesolution having a pH above 7 and in the presence of a phosphate, theelectrolyte solution also containing ethylene diamine tetramethylenephosphonic acid as a sequestering agent used in combination withtriethanolamine for binding any loose or superfluous ions so that theycannot react and form powder deposits.
 2. A method as claimed in claim1, wherein the phosphate includes an ortho-phosphate.
 3. A method asclaimed in claim 1, wherein the phosphate includes a pyro-phosphate. 4.A method as claimed in claim 1, wherein the phosphate is an alkali metalphosphate.
 5. A method as claimed in claim 1, wherein the phosphate isin the form of, or is provided by, a phosphoric acid.
 6. A method asclaimed in claim 1, wherein the pH is greater than
 9. 7. A method asclaimed in claim 1, wherein the pH is in the range of 10.2-11.0.
 8. Amethod as claimed in claim 1 wherein the electrolyte solutions containsan alkali metal hydroxide.
 9. A method as claimed in claim 8, whereinthe alkali metal hydroxide is KOH.
 10. A method as claimed in claim 1wherein the electrolyte further includes a plasma suppressing substance.11. A method as claimed in claim 10, wherein the plasma suppressingsubstance comprises a polyacrylamide.
 12. A method as claimed in claim1, wherein the sequestering agent comprises ethylene diaminetetramethylene phosphonic acid.
 13. A method as claimed in claim 1,wherein the current passed through the electrolyte solution is a pulsedDC current.
 14. A method as claimed in claim 1, wherein the currentpassed through the electrolyte solution is a continuous DC current. 15.A method as claimed in claim 1, wherein the anodizing of the magnesiummaterial follows a pre-treatment that prepares the magnesium materialfor anodization.
 16. A method as claimed in claim 1, further comprisingpretreating the magnesium material under conditions sufficient toprepare magnesium material for anodizing, and then anodizing saidpretreated magnesium material.
 17. A method as claimed in claim 16,wherein the pre-treatment includes at least one of the followingsub-step(s) selected from the group consisting of: (a) a cleaning step,(b) an etching step, and (c) a surface activation step.
 18. A method asclaimed in claim 17, further comprising carrying out a cleaning stepbefore and after the etching step.
 19. A method as claimed in claim 17,wherein the cleaning step includes an immersion of the magnesiummaterial into a solution containing caustic soda.
 20. A method asclaimed in claim 17, wherein the etching solution comprises at least oneacid.
 21. A method as claimed in claim 20, wherein the acid is at leastone member selected from the group consisting of nitric acid andphosphoric acid.
 22. A method as claimed in claim 17, wherein thesurface activation step includes immersion of the magnesium materialinto a solution containing a source of fluoride ions.
 23. A method asclaimed in claim 17, wherein the surface activation step includesimmersion of the magnesium material into a solution containing potassiumfluoride and at least one acid selected from the group consisting ofnitric acid and phosphoric acid.
 24. A method as claimed in claim 17,wherein the surface activation step includes immersion of the magnesiummaterial into a solution containing ammonium bifluoride.
 25. A method asclaimed in claim 17, further comprising rinsing said magnesium materialbetween each sub-step.